This worked in UE4 and provided a wonderfully flexible and powerful method of rendering scene component masks inside UMG - simply by assigning an image with a UI material that used a SceneTexture:CustomStencil node.
It was extremely performant and robust, and took very little setup to get working (unlike render targets).
Right now the SceneTexture:CustomStencil node outputs a constant value of 1 (or 0, I forget), please undo this regression, and make it a feature if it worked “accidentally”!